One of two methods, drop-on-demand or continuous, is typically employed in an apparatus that generates and selectively applies droplets. An example of such an apparatus is a continuous inkjet printer that generates and selectively applies droplets to a substrate to create a printed article. Further examples include inkjet-based computer-to-plate devices that generate and selectively apply droplets to a printing plate. Such apparatus either impart the necessary plate image printing characteristics required to successfully print, or produce a mask on a printing plate, which then undergoes additional operations to impart the necessary plate image printing characteristics that are required to successfully print.
A drop-on-demand apparatus, as its name implies, selectively generates droplets only when specifically needed. A continuous apparatus, on the other hand, continuously generates a stream of droplets from an uninterrupted stream of fluid, regardless of whether the droplets are specifically needed or not. Unlike a drop-on-demand apparatus, a continuous apparatus typically incorporates an ability to select specific droplets from the stream of droplets so that the selective application or selective use of these specific droplets can be accomplished. A common method of selecting these specific droplets from the rest of the droplets that are not required for subsequent application or subsequent use involves selectively charging some of the droplets and then using an electric field to discriminate between charged and non-charged droplets. Once selectively charged, either charged, or non-charged droplets may be applied to a substrate or used in some other application specific manner. In either case, charged droplets are deflected in an electric field, either to be applied to a substrate, or to be used in some application specific manner, or to be discarded into a disposal means typically referred to as a gutter.
In the case where charged droplets are applied to a substrate or used in some application specific manner, the charged droplets are deflected by an electric field to be applied or used, while the uncharged droplets maintain their original trajectory to be collected in a gutter. In this case, the amount of charge on the droplet determines the amount of movement of the droplets in the electric field. Such droplet movement may determine the relative position of the droplets that are applied to the substrate, for example.
In the case where the uncharged droplets are applied to a substrate or are used in some application specific manner, the charged droplets are deflected by an electric field into a gutter, while the uncharged droplets maintain their original trajectory to be applied to the substrate or to be used in some further fashion.
Typical continuous apparatus are equipped with a droplet generator that creates a stream of droplets. One type of droplet generator used in a typical continuous apparatus converts a continuous filament of fluid into a continuous stream of droplets. Various methods exist and are employed to change a continuous filament of fluid into a continuous stream of droplets. Most often such methods involve the application of an electrical stimulation signal to a suitable transducer in order effect some form of natural oscillation in the liquid, thereby facilitating the breakup of the liquid filament into individual droplets. It is common practice to employ a sinusoidal electrical signal of fixed wavelength for this purpose.
The stream of fluid breaks up into individual droplets at a distance (or time) from the point of origin of the stream of fluid commonly referred to as the “break-off point”. This break-off point is dependent on a number of parameters, including velocity, temperature, and fluid viscosity.
To create the appropriate charge on the droplets, the signal used for charging the droplets is usually applied to the stream of fluid before the moment the droplet separates from the stream, and held until the droplet is free of the stream. It is therefore clear that the phase relationship between the droplet stimulation signal and the droplet charging signal helps to determine the charge levels on the droplets.
The prior art describes a variety of ways to establish and control this phase relationship. One category of devices seeks to determine maximum charging of droplets by monitoring the current consumed by the droplets as they break-off from the drop generator. A second category of devices is based on sensing in a variety of ways the charge on either individual droplets or streams of droplets somewhere along their path of travel or at some collection point.
Yet a further family of methods employs a calibration signal that is applied to the droplets, typically at the charge electrode. In the prior art, this calibration signal is required to have a fixed phase or timing relationship with the charging signal or the droplet stimulation signal. The signature of the calibration signal produced by the droplets at some later point along their path, most typically measured at the droplet collection point or induced in a sensor of some form, is analyzed, and the relationship between the signature signal and the original calibration signal is then used to control the optimum droplet charging conditions.
The prior art systems which make use of calibration signals to control the charge levels on droplets make a specific point of applying the calibration signal selectively to specific droplets, depending on whether they are to be used for printing or not.
The prior art systems which make use of calibration signals to control the charge levels on droplets all share a common problem, in that they require complex timing arrangements in order establish the exact relationship between the calibration signal and droplet charging signal. There therefore remains a need for a simple and reliable means to control the charging of a stream of droplets.